This invention relates to articles having patterns of alternating hydrophilic and hydrophobic regions on the surface, and methods for making same.
Anti-dew coatings typically have hydrophilic surfaces that spread water droplets into a thin layer over the surface of the coatings. Anti-dew coatings have a variety of uses, such as on glass or plastic surfaces in greenhouses or indoor pools, and are especially useful on traffic signs that employ retroreflective sheeting.
Retroreflective sheeting has the ability to return a substantial portion of incident light in the direction from which the light originated. This unique ability has made retroreflective sheetings commonplace in traffic signs. Light from motor vehicle headlights is retroreflected by the sheeting to display information on the sign to the motor vehicle drivers.
Light transmission to and from a retroreflective traffic sign can be impaired by water droplets on the surface of the traffic sign. A prominent form of precipitation that affects light transmission is dew formation, which can be particularly problematic because it occurs predominantly at nighttime when the retroreflective sheetings are operative. Water droplets on traffics sign can significantly alter the path of incident and retroreflected light. This can make information on the sign much more difficult for passing motorists to read. Thus, the elimination or reduction of small beaded water droplets on the surface of a sign increases retroreflectance and readability by reducing the extent to which incident light is scattered or otherwise misdirected by water droplets on the surface of a sign.
To hamper water droplet formation in moist conditions, coatings have been applied to signs to evenly spread the water over the coating. Water-spreading coatings typically include inorganic particles and may also include an organic binder. For example: U.S. Pat. No. 4,576,864 to Krautter et al. discloses a water-spreading layer that is composed of colloidal particles of a metal or silicon oxide in which the water-spreading layer is adhered to a plastic substrate by an adhesive; U.S. Pat. No. 4,478,909 to Taniguchi et al. and U.S. Pat. No. 5,134,021 to Hosono et al. discloses an anti-fogging coating having finely divided silica particles dispersed in a matrix of polyvinyl alcohol and an organosilicon alkoxy compound or hydrolysates thereof; U.S. Pat. No. 4,409,285 to Swerdlow discloses a water-spreading coating comprising a mixture of large and small inorganic particles comprising colloidal silica and/or alumina; U.S. Pat. No. 4,481,254 to Fukushima et al. discloses an agricultural plastic film comprising an olefin resin and an amorphous hydrated aluminum silicate gel; U.K. Patent Application GB 2,249,041A to the Imperial College of Science, Technology and Medicine, discloses a modified hydrophobic plastic surface that has been subjected to an oxidation treatment and has a surface layer of colloidal hydrous metal oxide; Japanese Patent Kokai Publication No. HEI-3-50288 to Yamagishi et al. discloses an anti-fogging composition comprising a mixture of positively charged colloidal silica and alumina particles with a water-soluble aluminum salt and a nonionic surfactant; and U.S. Pat. Nos. 5,073,404, 4,844,946 and 4,755,425 to Huang disclose a retroreflective sheeting that has a transparent coating comprising colloidal silica and a polymer selected from aliphatic polyurethanes, polyvinyl chloride copolymers and acrylic polymers.
Other water-spreading layers are known that do not require inorganic particles. For example, U.S. Pat. No. 5,244,935 to Oshibe et al. discloses an ultraviolet curable anti-fogging composition agent comprising an acrylate or acrylamide block copolymer having a hydrophilic polymer segment and a hydrophobic polymer segment, a photopolymerizable compound, and a photoinitiator. The photopolymerizable compound has the formula CH2xe2x95x90CHCOO(CH2CRHO)nOCCRxe2x95x90CH2; when n=0, anti-fogging properties were not exhibited and when n greater than 30, the resulting film was weak. U.S. Pat. No. 5,316,825 to Nakai et al. discloses an anti-fogging film made of a transparent synthetic resin having micro concavities of at most 10 xcexcm in depth and 20 xcexcm in width.
Other workers have reported that anti-fogging properties can be imparted to glass or surface-activated plastic substrates by reacting the substrate surfaces with silanol or siloxane-functionalized polymers or fluoropolymers. European Patent Application No. 0 620 255 A1 to Luckey, Ltd. reports that anti-fogging coatings can be produced from mixtures of an epoxy functionalized organosiloxane, an amino functionalized organosiloxane, a hydrophilic methacrylate monomer, and a curing catalyst. U.S. Pat. No. 5,270,080 to Mino et al. discloses anti-fogging compositions composed of silanol-functionalized fluoropolymers. European Patent Application Nos. 0 491 251 A1 and 0 492 575 A2 to Matsushita Electric Industrial Co. report water-repelling, oil-repelling anti-fogging films that are made from siloxy-functionalized hydrophobic compounds. These references report that plastic surfaces can be made reactive to hydroxyl groups or hydrophilic by corona treating the surface.
Other techniques have resulted in heterogeneous surfaces. U.S. Pat. No. 4,536,420 to Rickert discloses a water-wettable coating made from a mixture of colloidal acrylic resin and colloidal silica which, when cured, has a mud-cracked pattern, thus providing canals in the surface which tend to break up water droplets. Japanese Kokai Patent Publication 59-176,329 to Mitsubishi Monsanto Kasei Vinyl K.K. discloses transparent molded materials having patterned surfaces of hydrophilic and hydrophobic areas. In the examples, a patterned hydrophobic material is printed onto a hydrophilic film.
In a first embodiment, the present invention provides an article having a patterned surface in which the article comprises a polymer having inorganic oxide particles dispersed throughout the polymer. The patterned surface of this article has alternating regions of relatively high and low concentrations of inorganic oxide particles on the surface. The regions that have a relatively greater concentration of surface inorganic oxide particles are hydrophilic, while the regions that have relatively few or no inorganic oxide particles on the surface are hydrophobic. In another embodiment, the invention comprises alternating hydrophilic and hydrophobic surface regions where the hydrophilic surface region has inorganic oxide particles exposed to the atmosphere and the hydrophobic surface region is essentially without inorganic oxide particles. In yet another embodiment, the invention provides an article having dew and/or frost resistance in which the article has a patterned hydrophobic/hydrophilic surface in which the hydrophobic surface regions are sufficiently narrow to inhibit the growth of dew droplets and/or inhibit the formation of frost.
The invention further provides methods for making an article that has a patterned surface that contains hydrophobic and hydrophilic regions. In one method, a coating mixture of inorganic oxide particles in a polymer precursor composition is coated on a base film that has a grooved surface. The coating mixture is deposited only in the grooves and does not cover the entire surface. In a preferred embodiment, the entire surface is then treated with a treatment that removes polymer, such as a corona or oxygen plasma, to expose inorganic oxide particles, thus forming a hydrophilic/hydrophobic patterned surface in which the surface of the coating mixture is hydrophilic and the uncoated tops of grooves remain hydrophobic.
In another method, inorganic oxide particles are dispersed in a polymer matrix that is formed into an article, typically a sheet or coating. A mask such as a screen is then placed over the article such that the mask protects selected regions of the surface while unprotected regions of the surface are exposed to the treatment. The surface is then exposed to a treatment that selectively removes the polymer matrix to expose oxide particles in the unprotected region. The region that is protected by the screen maintains a hydrophobic polymer surface. Thus, this method produces a patterned surface that matches the pattern of the screen.
Various embodiments of methods and products of the invention offer numerous advantages and uses. In preferred embodiments, the inventive article is dew and/or frost resistant; this means that the articles decrease the tendency of dew or frost to form. For example, in preferred embodiments where the article is a coating, the coated substrate exhibits a decreased tendency to become fogged or frosted relative to the uncoated substrate. In dew conditions, moisture in air is attracted to the hydrophilic areas of the patterned surface while the hydrophobic areas stay clear of condensation. In frost conditions, the hydrophobic areas of the patterned surface may remain relatively frost free, thus permitting light transmission with decreased diffraction through the article, and tending to break up ice formation between hydrophilic regions. In frost conditions, where the inventive article is disposed over a retroreflective substrate, the hydrophilic areas may tend to become frosted over; however, the hydrophobic areas remain relatively frost free thus allowing incoming light to be retroreflected back to the source.
The inventive articles can also offer optical properties such as transparency, economy, abrasion resistance, and durability including resistance to water damage and fungus. Thus, the invention is advantageous when used on retroflective signs, retroreflectors in pavement markers, face plates such as in surgical or scuba masks, and windows. The articles can be lightweight and can protect an underlying substrate. Articles of the invention that utilize fluoropolymer matrices also can offer anti-graffiti properties, can be oil and stain resistant, and can be cleaned relatively easily and economically.
The methods of the invention allow the surface to be modified in a highly regular and controlled fashion and to be shaped or oriented in a manner desirable for a particular end use. Unlike conventional printing methods, the methods of the invention preferably utilize a patterned surface region that is only a few micrometers (em) in width. In preferred embodiments, the invention advantageously provides patterned films or coatings that are capable of being made in a single coating step from the same composition.